Conventionally, 1,6-hexanediol has been prepared by preparing a carboxylic acid mixture containing glutaric acid, adipic acid and 6-hydroxycaproic acid, which is obtained as a by-product in oxidation of cyclohexane to cyclohexanone and/or cyclohexanol with oxygen; esterifying the carboxylic acid mixture; and then hydrogenating the esters to prepare 1,6-hexanediol, which is further separated by distillation (Patent Document 1).
However, 1,6-hexanediol obtained by this preparation process involves the problems that it contains an esterified product extremely difficult to be separated by common distillation and therefore has a higher ester value (EV), disadvantageously. Further, the following problems are involved in 1,6-hexanediol obtained by this preparation process. For example, the polymerization rate is low and the resulting molecular weight is not sufficiently high when polycarbonate diol prepared from the aforementioned 1,6-hexanediol is subjected to urethanation. The same problems occur when the 1,6-hexanediol is used for urethanation as-is as a chain extender (Patent Document 2).
A process for preparing high purity 1,6-hexanediol is also known in which 1,4-dihydroxycyclohexane is removed as an impurity by esterifying a carboxylic acid mixture obtained by oxidation of cyclohexane with oxygen, and purifying the esters by distillation (Patent Document 3). However, ε-caprolactone or 6-hydroxycaproic acid ester is hardly removed by this purification process, because it shows irregular behavior that cannot be expected from vapor pressure in the purification step by distillation.
It is also known that ε-caprolactone or 6-hydroxycaproic acid ester present in a 1,6-hexanediol mixture obtained by hydrogenation of an esterified product can be separated by saponifying with caustic soda and then purifying by distillation (Patent Document 4). However, the bottom residue containing 1,6-hexanediol in the distillation after the saponification is solidified or becomes more viscous, and is difficult to be recycled for distillation or the like, and finally causes a reduction in efficiency of preparation of 1,6-hexanediol.    Patent Document 1: U.S. Pat. No. 3,268,588    Patent Document 2: Japanese Unexamined Patent Publication No. 2001-316312    Patent Document 3: Japanese Unexamined International Publication No. 2000-505468    Patent Document 4: Japanese Unexamined Patent Publication No. 1103-115236    Non-Patent Document 1: Ullmann's Encyclopedia of Industrial Chemistry, 5. Ed, 1987. Vol. A8, S. 2/9    Non-Patent Document 2: Houben-Weyl, Methoden der Organischen Chemie, Band IV/Ic, Georg Thieme Verlag Stuttgart, 1980. S. 4567    Non-Patent Document 3: Houben-Weyl, Methoden der Organischen Chemie, Band IV/1c, S. 16-26